1. Field of the Invention
This invention relates to a structure for antenna, and more especially, to the antenna configuration for the access point (AP) adapted to the wireless local-area network (WLAN) or wireless metropolitan area network (WMAN).
2. Description of the Prior Art
Wireless communication systems have been developed rapidly. No matter in the business or in the family, the wireless communication systems are everywhere in people's life and are widely employed to provide various types of communication such as voice, data, and so on.
A multiple-input multiple-output (MIMO) communication system employs multiple transmit antennas and multiple receive antennas for transmission and reception of spatial-multiplexing data streams. In a point-to-point system, the data streams are transmitted to or received from a single terminal. However, a multiple access communication system having a base station may also concurrently communicate with a number of terminals. In this case, the base station employs multiple antennas to transmit or receive spatially multiplexed data streams to or from each terminal; each terminal on the other hand, employs multiple antennas to receive or transmit spatially multiplexed data streams from or to base station.
The advantage of the MIMO wireless systems is that the capability of the wireless link between the transmitter and receiver is improved compare with previous systems in the respect that higher data rates can be obtained. That is, higher spectral efficiencies are achieved than with non-MIMO systems.
Considering diversity gain, which is defined by:(Ideal Diversity Gain)×(1−ρ)(1/2),where the Ideal Diversity Gain is proportional to the dimensions n×m, n or m, wherein m for Transmit diversity gain, n for receive diversity gain, n×m for total system diversity gain. The correlation coefficient p which should be much less than unity is a function of: (1) separated antenna patterns (angular separation); (2) separated antenna positions (spatial separation); (3) isotropic distribution of incoming multipath waves (angular spread); (4) evenly-dispersive distribution of incoming multipath waves (delay spread).
Spatial Multiplexing technology is used for enhancing the transmission rate of the MIMO system. The spatial multiplexing gain that relates to throughput enhancement depends on orthogonality condition of MIMO antennas. In line of sight (LOS) or non-scattering MIMO environment or outdoor area, orthogonality condition is:St×Sr/R≧λ/M,where St and Sr are transmit and receive antenna spacings respectively, R is the range from transmit antennas to receive antennas, M is the number of receive antennas, the transmit antenna number N is not used in this condition.
As examples of access point (AP) and laptop PC, let F=5 GHz or λ=0.06 m, R=100 m, then                St≧12.5 m or 208 λ, if M=2 and Sr=0.24 m;        St≧25 m or 417 λ, if M=4 and Sr=0.06 m.        
In general, we can set 100 λ<St as a design rule in outdoor MIMO environment.
In none line of sight (NLOS) or scattering MIMO environment or indoor area, orthogonality condition is:[2×Dt/(N−1)]×[2×Dr/(M−1)]≧R×λ/M,where Dt and Dr are transmit and receive scattering radii respectively, R is the range from transmit scattering center to receive scattering center, N and M are the numbers of transmit and receive antennas respectively.
The scattering is made by scatterers in MIMO environment, which can be modeled by omni-directional ideal reflectors. The scatterers are assumed to be located sufficiently far from antennas for holding plane-wave assumption and further assumed such that Dt (or Dr) is much less than R for meeting local scattering condition.
As examples of AP and laptop PC, let F=5 GHz or λ=0.06 m, R=100 m, Dr=Dt, then                Dt=Dr≧0.866 m or 14.4 λ, if N=M=2;        Dt=Dr≧1.061 m or 17.7 λ, if N=2≠M=4;        Dt=Dr≧1.500 m or 25.0 λ, if N=4≠M=2;        Dt=Dr≧1.837 m or 30.6 λ, if N=M=4.        
In general, St<Dt. We can set 1 λ≦St≦10λ as a design rule in indoor MIMO environment.
According to the design rule, considering a device with a rectangular housing and 4 antennas, which can be used in 4×4 MIMO antenna system as base station or AP, there are several types of structure already known. FIG. 1 shows a co-linear antenna structure. There are four dipole antennas 2 connect to the AP housing 1, and the four dipole antennas 2 are align to the long side of the housing 1. FIG. 2 shows another co-linear antenna structure with the four dipole antennas 2 replaced by four Planar Inverted F Antennas 3(PIFAs). FIG. 3 shows a vertically coplanar antenna structure, where there are two dipole antennas 2 stands vertically by the two sides and two PIFAs 3 located within the housing 1. Alternatively, FIG. 4 shows another type of vertically coplanar antenna structure including two PIFAs 3 that are positioned inside the housing 1 at two corners of housing 1 and two dipole antennas 2 that stands vertically between the PIFAs 3 with equally spacing between the PIFA 3 and dipole antenna 2.
Please refer to FIG. 5, it shows alternative vertically coplanar antenna structure with two dipole antennas 2 and two PIFAs 3 aligned interlocked along the long side of housing 1. FIG. 6 illustrates another vertically coplanar antenna structure, where the two dipole antennas 2 and two PIFAs 3 positioned separately by the long side of the housing 1. Referring to FIG. 7, it shows a vertically coplanar antenna structure, where there are four dipole antennas 2 stands vertically by the four corners of the housing 1. Similarly, FIG. 8 shows another horizontally coplanar antenna structure, where there are four PIFAs 3 positioned at four corners of the housing 1. Alternatively, FIG. 9 shows a slant cubical antenna structure, where the two dipole antennas 2 stand vertically at two corners of one long side and the two PIFAs 3 stands at the other corners by the other long side of the housing 1. FIG. 10 shows askew cubical antenna structure, in the configuration, two dipole antennas 2 and two PIFAs 3 stands interlaced at four corners of the housing 1. FIG. 11 shows a concave cubical antenna structure, where there are three dipole antennas 2 stands vertically and forms a triangle at top surface of the housing 1, and the PIFA 3 positioned ant the center of the triangle inside of the housing 1.
The disadvantage of the antenna structures of the 4×4 MIMO system shown from FIG. 1 to FIG. 11 is that the efficiency of the system is poor. Furthermore, the system is more complex in mechanics and the cost is higher. What is required is a novel structure of MIMO antenna system to optimums the mechanics and cost.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description of the following detailed description given with reference to the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.